If Blender API document doesn't provide enough information of a function or class, what should we do?
The API is for the programmatic options and things.  What is happening in that line of code is a new node is being created of the type 'R_LAYERS' in the Compositor. In short, it just makes a Render Layers node. To know more about the node, you would need to look in the manual, not the API. Render Layers Node